The present invention relates to a pulse compression filter designed as a dispersive delay line which functions with acoustic waves and has a prescribed center frequency which produces a corresponding mean wave length in the filter. A transducer for either the input or output typically has a plurality of electrode fingers and wherein a distribution of the fingers corresponds to a prescribed transfer function.
Pulse compression filters with which a pulse of short time duration can be transformed into a pulse of duration T repesenting a long time duration are particularly employed in radar technology. This is generally carried out at the transmit side. Presuming that a matched filter is employed at the receive side, the pulse which has been time expanded at the transmit side with a pulse comression filter can be converted back into a short pulse with a time duration greater than and/or equal to 1/B. This reconverted pulse is necessarily accompanied by side lobes with a more or less slight amplitude level which depend on the prescribed filter transfer function and the product of the duraction T of the expanded pulse and the bandwidth B of the filter (=T.multidot.B).
The technique of pulse expansion at the transmit side is particularly employed in order to be able to transmit a radar pulse with a high energy content given a limited transmit power. By means of compression again undertaken at the receive side, the high time resolution can be approximately retained which would be achieved given a transmission of the original, short pulse. The higher pulse energy makes a greater range possible for a radar system equipped with such a pulse expansion/compression.
The theoretically based advantages of pulse expansion/compression are not entirely achieved in practice, because, among other things, of the limited technical expense of the filter. The time side lobes, however, occur not only because of the limited value of the product T.multidot.B of the pulse compression filters but, rather, they also appear given practically employed filters which have tolerances associated with the topical disposition of the electrode fingers.
Such fingers have associated therewith a piezo-electrical substrate on whose surface the electrode fingers are disposed in a distribution prescribed by the transfer function. Bus bars are selectively connected to the electrode fingers.
There are various types of pulse compression filters. One such type is designed as a tapped phase-encoded delay line. It contains electrode fingers in a disposition such that electrode fingers combined in groups which are connected to the one or to the other feed electrode (bus bar) have a successively different phase, whereby the succession of different phases of the group follows a prescribed encoding. Such a phase-encoded line has the disadvantage that relatively high side lobes whose minimum height is fixed by the number of phase shifts in the transmitted pulse occur in the compressed pulse. Moreover, such a line also has the disadvantage that it is very sensitive to frequency shifts in the pulse signal and/or in the line, for example due to the Doppler effect or due to a temperature of the substrate.
Such disadvantages are avoided given a pulse compression filter designed as a dispersive delay line. Such a line, however,has the disadvantage on the other hand that its mutually adjacent electrode fingers have spacings from one another which continuously differ, for example, a spacing which increases of decreases continuously from electrode finger to electrode finger. This causes technological manufacturing difficulties because such a filter can easily have a length of 10 to 15 cm. The values for the width dimension and the spacing of the electrode fingers from one another go down to a size of 1 .mu.m. Tolerances of up to less than 0.1 .mu.m must be observed for the precision of the limitations and the spacings of the electrode fingers in order to avoid disruptive side lobes of the recompressed pluse. It is advantageous to manufacture such structures photolithographically with a reducing projection of a master on the substrate. In practice, however, projection means which are sufficiently distortion-free are only available in which the diameter of the image field amounts only to a fraction of the length of the finger structure. One is therefore forced to manufacture the entire electrode finger structure with a continuously changing electrode finger spacing composed of a plurality of sections.
It has been shown that support/forward feeds for projcetion devices are definitely available which allow the relative feed of the substrate and master to be realized with high precision. If, however, one must change the master, as is required given an electrode finger spacing which constantly changes, then imprecisions in the electrode finger structure result which cannot be tolerated. One way of avoiding the difficulty described above is that each individual electrode finger of the filter or respectively of the line, is manufactured by means of individual exposure, i.e. by an individual technique. The time for executing such a method is not economical for the manufacture of a pulse compression filter with such an electrode finger structure.
Only for the sake of completeness we should like to point out the method of copying with contact printing of a mask. This, however, is less advantageous because of the contamination of the substrate surface which is connected therewith.